BHR Hollywood Reports........The "Blurred Lines" crooner and his girlfriend April Love Geary announced that they are pregnant with their first child.

22-year-old April Love Geary is 3 months pregnant with a girl ... the baby is due March 1.
40-year-old Robin has been dating April for 3 years ... shortly after his wife, Paula Patton﻿, filed for divorce.

Robin and Paula are still not divorced, and they have had an on-and-off custody war over their 7-year-old son, Julian.

By the way, we're told Robin and April told Julian this week and he's "super excited for a little sister."